1. Field of the Invention
The present invention relates to a permanent magnet motor provided with an R—Fe—B based rare-earth sintered magnet that includes crystal grains of an R2Fe14B type compound (where R is a rare-earth element) as a main phase.
2. Description of the Related Art
An R—Fe—B based rare-earth sintered magnet, including an R2Fe14B type compound as a main phase, is known as a permanent magnet with the highest performance, and has been used in various types of motors such as a voice coil motor (VCM) for a hard disk drive and a motor for a hybrid vehicle and in numerous types of consumer electronic appliances. When used in motors and the like, the R—Fe—B based rare-earth sintered magnet should exhibit thermal resistance and coercivity (HcJ) that are high enough to withstand an operating environment at an elevated temperature.
However, the remanence (Br) and the coercivity (HcJ) are generally conflicting properties: when it is intended to increase the coercivity of the entire permanent magnet, the remanence of the entire permanent magnet will relatively decrease.
An alloy in which a light rare-earth element RL such as Nd is partly replaced with a heavy rare-earth element RH such as Dy and Tb is conventionally used to increase the coercivity (HcJ). However, with the conventional technique, the decrease in remanence (Br) is still unavoidable. It should also be noted that use of the heavy rare-earth element RH such as Dy and Tb, which is a rare metal, is preferably cut down as much as possible.
To address the problem described above, Japanese Laid-Open Patent Publication No. 57-148566 and Japanese Laid-Open Patent Publication No. 8-340651 disclose that the coercivity (HcJ) could be selectively increased for the portion of a permanent magnet used in a motor, on which a large demagnetizing field acts due to a temperature rise, armature reaction and the like during operation of the motor. According to this technique, a permanent magnet with an especially enhanced coercivity (HcJ) is joined to another permanent magnet with an adhesive or the like.
The inventions disclosed in Japanese Laid-Open Patent Publication No. 57-148566 and Japanese Laid-Open Patent Publication No. 8-340651 may produce some degree of effect because the decrease in the remanence of the permanent magnet as a whole is minimized. However, the remanence greatly decreases in the portion of the magnet with an enhanced coercivity, compared with the other portion thereof. For this reason, the remanence sharply changes in the enhanced-coercivity portion, and hence when used in a motor and the like, the magnet will cause problems such as decrease in torque, increase in torque ripple and resultant occurrence of noise.